Automatic gain control circuit for electron multiplier phototubes



July 29,- 1952 H. o. MARCYNQI AUTOMATIC GAIN CONTROL C RCUIT FOR ELECTRON MULTIPLIER PHOTOTUBES Filed Feb. 20, 1946 vvvv INVENTOR. HENRY o. MARCY 111 UNGROUNDED 32' POWER SUPPLY AT TORNEY Patented July 29, 1952 PATENT OFFICE AUTOMATIC GAIN CONTROL GIRCUIT- Fort- I ELECTRON MULTIPLIER PHOTOTUBES" Henry 0. Marcy, 3rd, Park Ridge, 111., assignor, by"

'nre'sn 'assig men s, to the United States of i America as represented by the Secretary'of the,

' Application-February 20, inslseri'alnb.6.45.095.

This invention applies "to electronic. control systems and more particularly to automatic 'gain control systems for electron multiplier Iphototubes l v Heretofore the operation of electron multiplier phototubes has beenierratic such that in some instances the final stages or electron multiplication within the phototube saturated sothat the output of the phototube no longer distinguished between different light levels at the input.

The general object oi the present invention is to overcome the ioregoing'difliculties.

Another object of the present i'nvention'isf' to provide an automatic gain control system for electron multiplier photot'ubes so as to prevent saturation of any of the stages of the phototube.

These and other objects will'beapparent from the following specification when taken withfthe drawing, the" single figure of which is a'schematic diagram of one form of'the invention.

Referring 'to the drawing, an automatic gain control system for electron multiplier phototube II is shown having'an ungrounded source of voltage l3 and a variable resistance electron tube "connected in series with voltagesource I3 and phototube H. Electron tubes I8 and I9 are energized by the signal output of the phototube appearing at terminal 59. Tube [8 has in its cathode output circuit a/comparatively'long time constant circuit comprising resistor 41 and condenser 43, in parallel. The output from tube 19 is, coupled back to the grid of tube I! to control its grid, bias. This, in turn controls. the

voltages applied to the phototube I l and thus the gain thereof. Electron tubes and I6 in series form a cascade amplifier to amplity the signal output of phototube ll before it appears at output terminal 59. v

Electron multiplier phototube H has as its electrodes a light sensitive cathode 2|, agfinal anode or collector 29, and intermediate anodes or secondary emitters 22, 23, 24, 25, 26,21, and

28. It is to be understood that phototube [I need not be limited to the number of electrodes or to the particular multiplier structure asshown in the drawing. A bleeder comprising resistors 3], 32, 33, 34, 35, 35, 3.1, 38, and 39 in series is used to apply successively more positive voltages from voltage source l3 to successive electrodes of phototube ll, photoelectric cathode 21 being the most negative, anode22 being more positive, and i so. on'with final anode Zia-being most positive;

" "1 3Claims. (01350-207 v y i iffi2 v In electron multiplier;phototube ll the currentat the final anode;2;9 is a I i=iog where I y it is the current emitted by cathode 21,3 g is the ratio of secondary to primary electrons at each intermediate anode, 22 through 28, and n is, thenumber of such intermediate anodes.

Also rgl is proportional to the voltage between intermediate electrodes which in turn is dependent onthe voltage applied. across the bleeder made up ofresistors. 31 through 39. In the present invention, apparatusis provided to automatically control thiswvoltage. andthus the gain of phototube l I, Itis obvious that large changes of gaincan beobtained with small changes of voltage.

As an example of. the application ofthis invention in a. radar training device the: input to the phototubel I is a modulated light. source. It is necessary thatthe outputvoltage at. terminal 59 be a faithful reproduction of the. modulated light sourceiinput; If the final stages of phototube II should saturate all 'or part of the input modulation would be lost at the output.

To prevent any saturation: of thefinal'stages of phototube l I bleede'r resistors 3| through 39 are energized from. an 'ungrounded power:.sup ply; [3 through electron tubesili, l6, and 11' in series. Theij-unctionsof:resistors-31 and 32are also connectedflto a positive: potential. applied at I terminal 51 through resistorsi4j2r-and 43; inseries.

If the grid bias ofielectron tube.- H is: increased its plate resistancewillt increase and therefore the voltage across bleeder. resistors:,3l, through 39 will decreasereducing the gain of phototube I serve as a voltage divider, their junction being connected to thegrid of, tube lfisto determine its fixed bias and thus the operating points of tubes l5 and I6.

Final anodev 29 is'connected to the junction of resistors; 3| and 32 through resistor 4l which acts as-the loadresistor for phototube H. As

the current. at finalanode 29 increases due to an.

increased light input atcathode 2| the. voltage at anode; 29 becomes l'nii ie-negative, Since anode 9. i t r t r s n adtc he r of tub 15 A resistor 54, is; connected in. parallel with formrthe invention may take.

ground from a positive potential applied at ter-- minal 51, comprises a voltage divider to determine the fixed cathode bias of tube I8.

The grid of tube I8 is connected to the output voltage ape-:

pearing at potentiometer '40 which is so set that tube I8 will benon-conductingwith a very low level or zero light input to phototube I I. Tube I8 has its cathode connected to the grid of tube I9 and to a comparatively long time constant circuit comprising the parallel combination of resistor 4! and condenser 48. As the light input to phototube II increases tube I8 begins to conduct charging condenser'48 rapidly and raising the grid potential of tube I9. Thus the plate of tube I9 rapidly drops in potential. A voltage divider, comprising the series combination of resistors 50, 5I and 53 and potentiometer 52, is connected from a positive potential at terminal 51 to the negative terminal of power supply I3. The plate of tube I9 is connected to the junction of resistors 50 and =5I. The output of this circuit at the variable arm of potentiometer 52 is coupled back to the grid of .tube I! to control its bias. The resistors 50, 5|, andl53 are chosen and potentiometer 52 set so that with a low level light input .toithephototube-II. the grid of tube U will beat essentially ground potential and tube II will have a low plate. resistance. Larger light inputs will cause tube I! to increase its plate resistance. r

r In theabove electron tube circuit, tube I8 with resistor 41 and condenser 48 is essentially a peak detector when a modulated light input is used. It, with the other circuits, automatically controlsthe gain of phototube II on the basis of thestrongest light input so that no stages of phototube I I saturate. Tube I8 also serves the purpose of isolatingoutput terminal 59 from the long time constant circuit of resistor 41 and condenser 48. Tube I9 .isan amplifierand is self-biased by its cathode resistor 49. Condenser 55 is connected from the variable arm of potentiometer 52 to the negative terminal of power supply I3 vandpreventsfia'ny'rapid fluctuations in the gainof-phototube II. V

The invention described in the foregoing specification need not be limited to the details shown, whichrare considered to be illustrative of one It is to be understood thatthe-cascade' amplifier of tubes I5 and I6 maybe eliminated: or replaced by any other amplifier giving'thedesiredgain. It is also to be understood that the electron tube circuit (tubes I 8 I and I9) may readily be replaced by any other combination of electron tubes and a comparatively long time constant circuit.

What is claime'dis: v I

1. An automatic gain controlsystem for an electron; multiplier phototube comprising, a source of voltage for the'el'ectrodes of said phototube, means for connecting a variable resistance element in series with said voltage source and said electrodes, and means coupled between the output circuitof said electron multiplier phototube and said variable resistance and responsive trodes a light sensitive cathode, a final anode, and a plurality of intermediate anodes, said gain control circuit comprising, a source of potential having the negative terminal thereof connected to said-cathode, a potential divider including a plurality of terminals having successively more positive voltages when said divider is coupled 'to the positive terminal of said potential source,

means connecting said terminals to said cathode and to said plurality of intermediate anodes so that successive intermediate anodes are connected to terminals successively more positive than the preceding intermediate anode and more positive than said cathode,an output circuit coupled to said electron multiplier phototube, an electron tube having a variable plate resistance controlled by the grid bias applied to said tube, said tube being connected in series with said source of potential and said potential divider, and means coupled between said output circuit and the grid circuit of said electron tube for controlling the grid bias of said variable resistance electron tube in accordance with the amplitude of the'output signal from said phototube, thereby to control the voltages applied to said electrodes and the gain of said electron multiplier phototube.

3. The apparatus of claim 2 wherein the means for controlling the grid bias of said variable resistance electron tube comprises an electron tube circuit energized by the signal output of said phototube, said circuit comprising a second electron tube and a comparatively long time constant circuit, the output of said second electron tube being coupled back to the grid circuit of said first mentioned electron tube to control the gain of said electron multiplier phototube on the greater signal outputs thereof.

4; An automatic gain control circuit for an electron multiplier phototube having a cathode, a final anode, and a plurality of intermediate anodes, said control circuit comprising, a voltage dividing network connecting said cathode, said intermediate anodes and said final anode, an output circuit, coupled to said final anode, a voltage source; variable impedance means coupling said voltage source to said voltage dividing network, "and means coupled to said variable impedance means and said output circuit for altering the impedance of said variable impedance means "in accordance with the maximum amplitude of the'output signal from said phototube.

5. An automatic gain control circuit for an electron multiplier phototube having a cathode, a final anode, and a plurality of intermediate anodes, said control circuit comprising a load impedance coupled to said final anode, a voltage dividing network connecting said cathode, said intermediate anodes and said load impedance, a voltage source, variable impedance means coupling said voltage source to said dividing network,

peak detecting means coupled to said load imelectron multiplier phototube having a cathode, a finalanode, and a plurality of intermediate anodes, said control circuit comprising a load impedance coupled to said final anode, a voltage dividing network connecting said cathode, said intermediate anodes and said load impedance, a voltage source, an electron tube having a variable anode-cathode resistance controlled by the grid bias applied to said tube, said electron tube coupling said voltage source to said divider through said anode-cathode resistance, a peak detecting means coupled to signals appearing across said load impedance, said peak detecting means having the output thereof coupled to said electron tube to control the bias thereof.

7. An automatic gain'control circuit for an electron multiplier phototube having a cathode, a final anode and a plurality of intermediate anodes, said control circuit comprising, a load impedance coupled to said final anode, a voltage dividing network connecting saidcathode, said intermediate anodes and said load impedance, a voltage source, an electron tube having a variable anode-cathode resistance controlled by the grid bias applied to said tube, said electron tube coupling said voltage source to said divider through said anode-cathode resistance, an amplifier coupled to said load impedance, a peak detecting means coupled to said amplifier and having the output thereof coupled to said electron tube to control the bias thereof whereby the gain of said phototube is determined by the peak amplitude of signals in the output thereof.

8. Apparatus as in claim 7 wherein said peakdetecting means includes an output circuit having a time constant that is long compared to the duration of individual fluctuations in signals in the output of said phototube multiplier.

9. A gain control circuit as in claim 7 wherein the output of said peak detecting means is coupled to said electron tube through a second voltage dividing means, said second voltage dividing means including a means for preventing rapid variations in the bias of said electron tube.

10. An electron multiplier phototube amplifier stage with automatic gain control, said amplifier stage comprising, an electron multiplier phototube having a cathode, a final anode and a plurality of intermediate anodes, a voltage dividing network connecting said cathode, said intermediate anodes and said final anode, a voltage source, variable impedance means coupling said voltage source to said voltage dividing network, and means coupled between the output of said amplifier stage and said variable impedance means for altering the impedance of said variable impedance means in a direct relationship to the alteration of the maximum amplitude of the output signal from said amplifier stage occurring within a preselected time interval.

11. An automatic gain control circuit for an electron multiplier phototube having a cathode, a final anode, and a plurality of intermediate anodes, said control circuit comprising, a load impedance coupled to said final anode, a voltage dividing network connecting said cathode, said intermediate anodes and said load impedance, a voltage source, variable impedance means coupling said voltage source to said dividing network, detecting means including a triode electron tube and resistance-capacitance circuit connected in the anode-cathode circuit of said triode electron tube, means coupling the grid circuit of said triode electron tube to said load impedance and.

said cathode, said intermediate anodes and said load impedance, a voltage source, variable impedance means coupling said voltage source to said dividing network, detecting means coupled to said load impedance, said detecting means being adapted to produce an output signal pro-.

portional in amplitude to the amplitude of the amplified input signal, said detecting means having the output thereof coupled to said variable impedance means to control the impedance thereof in accordance with the amplitude of the output signal from said amplifier stage.

13. An electron multiplier phototube amplifier stage with automatic gain control, said amplifier stage comprising an electron multiplier phototube having a cathode, a final anode, and a plurality of intermediate anodes, the signal to be amplified being applied to said cathode of said phototube, a load impedance coupled to said final anode, a source of bias voltages coupled to said cathode, said intermediate anodes and said load impedance, detecting means coupled to said load impedance, said detecting means having the output thereof coupled to said source of bias v voltages to control the amplitude of said bias voltages in direct relationship with the changes in amplitude of the output signal from said amplifier stage- HENRY O. MARCY, 3RD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,810,739 Vedder June 16, 1931 1,883,926 Iams et a1 Oct. 25, 1932 2,074,030 Shoup Mar. 16, 1937 2,290,775 Snyder, Jr. July 21, 1942 2,412,423 Rajchman et al. Dec. 10, 1946 2,454,871 Gunderson Nov. 30, 1948 

